1. Field of the Invention
The invention relates to a method and apparatus for marking pipe and other hot formed metal products while they are still hot from fabrication. The method and apparatus are particularly useful for marking detected flaws in hot metal as it emerges from the mill.
2. Description of the Prior Art
In the manufacture of steel and other metal products it is desirable to inspect for flaws and imperfections. In current manufacturing processes most inspection is done by or with the assistance of some type of detecting apparatus. Usually one or more inspections are made immediately after the final fabrication and/or at final size.
In the manufacture of continuous welded pipe the weld seam is frequently inspected by an eddy current detector positioned near the output of the forming mill. Each detection assembly has one or more detecting coils which are inductively coupled to the workpiece and which effect the flaw detection. This detection results from the capability of a detection coil to produce output signals in response to variations in eddy current flow in the workpiece. These variations are caused by flaws in the workpiece. Accordingly, variations in output signals from the detector coils which trip a pre-set alarm level activate a marking device which deposits a marker on the workpiece. Welded pipe is at a temperature ranging from 1850.degree. F. to 2150.degree. F. If one attempted to use conventional markers such as paint, grease or even sulfuric acid the marker would vaporize as it nears the workpiece and make no mark on the pipe. The use of various cutting or grinding tools to physically remove some metal and form a mark in the surface of the workpiece has also been proposed. But use of a grinder or cutter is impractical for a pipe mill where the workpiece to be marked is a hot steel pipe traveling at at speeds of the order of 200 to 1000 feet per minute.
Vild et al. in U.S. Pat. Nos. 4,123,708 and 4,127,815 propose the use of an aluminum powder having a melting point of approximately 1200.degree. F. as a marker. When the powder is sprayed onto the hot workpiece which is at temperatures above the melting point of the aluminum powder, the aluminum fuses and provides a visible mark. Others have added various percentages of titanium dioxide to the aluminum powder to enhance the results.
Although the technique taught by Vild et al. will provide a visible mark on the pipe, that mark will not survive subsequent processing steps such as cooling, straightening and facing. Thus, if one uses Vild's system he must be able to remove marked pipe from the production line before subsequent processing. Many pipe mills are not equipped to remove pipe from the production line before it passes through cooling, straightening and facing operations. Hence, the Vild marking system is impractical for those plants.